DrummerHead/JS-Inject

From:
http://mcdlr.com/

See it live at:
http://mcdlr.com/js-inject/

With this tool creating a Bookmarklet is much easier. Just type the code, assign a name and an Anchor element with your code will be created below for you to drag to your bookmarks.

You also have the possibility of adding jQuery to the target page so you can write you javascript using jQuery, just a checkbox away!